TECTONIC EVOLUTION OF THE BREVARD FAULT ZONE ADJACENT TO THE GRANDFATHER MOUNTAIN WINDOW, NC

The Brevard Fault Zone adjacent to the Grandfather Mountain window, NC is interpreted to be a terrane boundary between basin and volcanic arc rocks amalgamated to the Laurentian margin during the Taconic orogen. The fault zone consists of a northwestward shallowing thrust stack involving a cataclastic to mylonitic backstop, the Alligator Back metamorphic suite, and thrusts propagating into the Grandfather Mountain Formation and Grenville basement.

Rocks to the NW of the fault zone, in the Grandfather Mountain window, exhibit both ductile and brittle deformation with annealed quartz matrix and fractured potassium feldspar and large quartz porphyroclasts that are sometimes visible at the macro scale. EBSD quartz CPO pole figures support a high temperature, ~550-600°C, deformation with a lower temperature overprint at ~350°C. Based on thermochronology dates from Thigpen et al., 2022, we interpret the earlier high-temperature event to have occurred during the Taconic, and possibly Early Acadian. The brittle deformation likely occurred during Alleghenian thrusting.

In contrast, the Tugaloo Terrane rocks SE of the fault zone exhibit ductile deformation structures. Although quartz CPO pole figures show a similar high and low-temperature history as the data to the NW, the porphyroclasts have tails or pressure shadows, and the quartz grains in the matrix exhibit dynamic recrystallization up to GBM (II). Fractured porphyroclasts are rare to nonexistent. We interpret this as consistent with being near the margin of an orogenic channel that prior thermochronology data supports being active during the Acadian orogen (Hatcher and Merschat, 2006). Overall, the incipient Brevard Fault zone was first activated as a terrane boundary with high-temperature deformation during the Taconic to Acadian orogeny, then reactivated as a buttress/ramp for Alleghenian thrusting.